For the starter, put sugar in warm water to dissolve.,Add yeast and allow to proof.,Juice the lemons and grate the rinds.,Put into a heatproof container with the sugar and ginger and pour in 1 cup boiling water.,Allow to steep for 10 minutes.,Strain into a 1.5 liter bottle and top up with cool water so it is room temperature.,______,Mix thoroughly and put in a warm place.,Leave the bottle alone until you cannot dent it (12 hours-3 days).,Be sure to check regularly.,Refrigerate until chilled and open carefully.